Although gastrin hormone was first identified one hundred years ago, and was purified in the 1960's, its effects on different tissues in normal and disease tissues is still incompletely understood. One major reason for this gap in knowledge of the gastrin system has been the difficulty in separately detecting and quantifying each of the several forms of gastrin hormone.
In mammals the peptide hormone, gastrin exists in several forms, grouped into two major size classes, “little” gastrin and “big” gastrin, on the basis of the number of amino acid residues in the peptide chain. The “little” gastrin form includes mature gastrin-17 (G17) and glycine-extended G17 (G17-Gly); and “big” gastrin includes gastrin-34 (G34) and glycine-extended G34 (G34-Gly). The mature form of G17 is a major effector of stomach acid secretion and is estimated to be about six times more effective in this role than is G34. The various forms of gastrin are produced in vivo from a precursor peptide, progastrin, by cleavage and in some cases, modification of the cleaved form. Human G34 has the entire seventeen amino acid sequence of G17 at its C-terminal, and, predictably, cross-reacts immunologically with G17.
Mature G17 is modified at both amino- and carboxy-terminal residues: the N-terminal glutamic acid is cyclized to form pyroglutamic acid (pGlu) and the free carboxyl group of the C-terminal phenylalanine residue is amidated by the enzyme, peptidyl α-amidating mono-oxygenase (PAM) to form a C-terminal Phe-NH2. (See Dockray et al., Ann. Rev. Physiol. (2001)63: 119–139).
Mature G17, the predominant form of “little” gastrin in humans, has the amino acid sequence: pEGPWLEEEEEAYGWMDF-NH2 (SEQ ID NO: 1). G17-Gly is an incompletely processed form of gastrin found as a minor component of “little” gastrin in healthy human subjects and has the amino acid sequence: pEGPWLEEEEEAYGWMDFG (SEQ ID NO: 2).
Gastrin-34, the predominant form of “big” gastrin in humans, has the amino acid sequence: pELGPQGPPHLVADPSKKEGPWLEEEEEAYGWMDF-NH2 (SEQ ID NO: 3), and glycine-extended gastrin 34 (G34-Gly), has an extra C-terminal glycine residue, having the amino acid sequence: pELGPQGPPHLVADPSKKEGPWLEEEEEAYGWMDFG (SEQ ID NO: 4).
Gastrin is secreted by the pyloric antral-G cells of the stomach in response to gastrin-releasing peptide (GRP). Gastrin secretion is suppressed by gastric acid and the paracrine action of several peptide hormones, most notably, somatostatin. It has long been recognized that gastrin peptides function to stimulate acid secretion in the stomach of healthy individuals, however, it has only recently been shown that these peptides also control proliferation, differentiation and maturation of different cell types in the gastrointestinal (GI) system.
In addition to their local activity in the GI system, G17 and, to a lesser extent, G17-Gly are released into the bloodstream and have been found to increase in the serum of patients afflicted with gastrointestinal disorders and diseases, such as gastric cancer, colorectal cancer, and pancreatic cancer. These gastrin species have more recently also been found to be associated with other diseases not associated with the gastrointestinal tract, including small cell lung cancer (SCLC) and liver metastasized tumors. See for example “Gastrin and Colon Cancer: a unifying hypothesis” S. N. Joshi et al., Digestive Diseases (1996) 14: 334–344; and “Gastrin and colorectal cancer” Smith, A. M. and Watson, S. A. Alimentary Pharmacology and Therapeutics (2000) 14(10): 1231–1247.
Antibodies are key reagents in numerous assay techniques used in medical, veterinary and other fields. Such tests include many routinely used immunoassay techniques, such as for example, enzyme-linked immunosorbant assays (ELISA), radioimmunoassays (RIA), immunohistochemistry (IHC), and immunofluorescence (IF) assays.
Monoclonal antibodies (MAbs) have unique characteristics that render them superior in many respects to polyclonal antisera and to antibodies purified from polyclonal antisera when used in many of these assays. These attributes include monodeterminant specificity for the target antigen (i.e. specificity for a single epitope), unchanging specificity among different antibody preparations, as well as unchanging affinity and chemical composition over time. Furthermore, MAbs can be produced indefinitely and in unlimited amounts by in vitro methods. These properties are in sharp contrast to those of polyclonal antibodies, which require in vivo immunization methods with the unavoidable associated biological variability and limited antibody production capacity over the lifespan of the immunized animal.
Despite these advantages, differences exist between individual MAbs even though they may be specific for the same epitope. For example, differences between MAbs induced by immunization with a single antigenic epitope region can arise with respect to any or all of the following characteristics: 1) the fine specificity for the molecular composition and tertiary structure of the epitope; 2) the antibody idiotype; 3) the antibody affinity; 4) the antibody allotype; and 5) the antibody isotype. These characteristic differences can affect the behavior of MAbs in a particular immunoassay, such that different MAb isolates raised against the same antigenic region can behave differently in a given assay. Consequently, some MAbs will be superior to others that bind the same epitope when used as reagents in a particular immunoassay.
The immunoassay may be an enzyme-linked immunosorbent assay (ELISA) or a radioimmunoassay (RIA), an immuno-detection assay, such as an ELISPOT, slot-blot, and western blot. As a general guide to such techniques, see for instance, Ausubel et al. (eds) (1987) in “Current Protocols in Molecular Biology” John Wiley and Sons, New York, N.Y. Alternatively, the immunoassay may be an immunohistochemical (IHC) staining or immunofluorescence (IF) procedure for visualization of a gastrin hormone in a tissue sample. See for example “Principles and Practice of Immunoassay” (1991) Christopher P. Price and David J. Neoman (eds), Stockton Press, New York, N.Y.
Monoclonal antibodies specific for the N-terminal region and the C-terminal region of G17 have been described. See for example, Azuma et al., Gastroenterologica Japonica (1986) 21(4): 319–324; Ohning et al., Peptides (1994) 15(3):417–423; Fuerle et al., Pancreas (1995) 10(3):281–286; Kovacs et al., Peptides (1996) 17 (4): 583–587; Ohning et al., Am. J. Physiol. (1996) 271(3 Pt 1):G470–476; Sipponen et al., (2002) Scand. J. Gastroenterol. 37(7): 785–791. However, none of these antibodies were shown, either alone or in combination, to be capable of distinguishing and quantifying each of the forms of gastrin hormone found in biological fluids in normal and disease states.
Anti-gastrin polyclonal antibodies have been shown to be effective in inhibiting gastrin activity (“Inhibition of gastrin activity by incubation with antibodies to the C-terminal tetrapeptide of gastrin” Jaffe et al., Surgery (1969) 65(4):633–639); and non-human anti-gastrin polyclonal antibodies have been applied to therapy in a patient suffering from Zollinger-Ellison syndrome, a pathological condition in which excessive gastrin is produced without stimulation by feeding. See Hughes et al., “Therapy with Gastrin Antibody in the Zollinger-Ellison Syndrome” Hughes et al., Digestive Diseases (1976) 21(3):201–204. However, these rabbit anti-gastrin antibodies had “at best a short term effect in this patient.” (Hughes at p. 204).
Recently, the ratio of amidated to non-amidated gastrin hormone in serum has been suggested as providing an indication of an individual's risk profile for developing duodenal ulcer disease or gastric atrophy. See published U.S. patent application 2003/0049689 entitled “Diagnosis and Treatment of Gastrointestinal Disease” of T. C. Wang.
Until now, MAbs capable of sensitively detecting, and accurately distinguishing each of the G17, G17-Gly, G34, and G34-Gly forms of gastrin hormone have not been available. Furthermore, until the present invention, it was not possible to accurately measure the amounts of each of these forms of gastrin hormone in a sample of biological fluid. Use of the Mabs of the invention in assays for clinical testing more precisely defines the biology of gastrin hormone in normal and disease states and to provides MAb compositions for pharmaceutical use and methods for the prevention and treatment of gastrin-associated diseases and conditions.